The present invention relates to a multilayer hollow body made of a thermoplastic material, which is impermeable to liquids and/or to gases.
Hollow bodies, particularly tanks used at the present time for storing liquids and/or gases often consist essentially of plastic because of weight, mechanical strength, chemical resistance and easy processibility reasons, particularly when they have complex external shapes.
Plastic tanks are widely used in motor vehicles, where they are employed for containing various fluids: brake fluids, liquids for washing windscreens and headlights, and various fuels (diesel, petrol, LPG gas).
It is often imperative, for motor-vehicle applications, to ensure that the tanks on board are impermeable, particularly in the case of fuel tanks.
Various techniques are known for impermeabilizing the surfaces of plastic articles. Two different approaches have led to solutions being proposed which have been employed in processes for manufacturing these articles. The first is based on the surface treatment of the article by means of a plasma or by means of a chemical reactant, such as sulphur dioxide or fluorine gas, which is capable of modifying the molecules of the plastic which lie at the surface of the article. Another approach consists in sandwiching, into the thickness of the plastic of which the article is composed, a layer comprising a specific material, often thermoplastic, having a barrier property with respect to one or more gases or liquids. In the latter case, co-extrusion processing techniques have been widely employed.
As barrier material, it is common practice to use fluorinated polymers, polyamides, polyacrylo-nitriles, polyesters and polyvinylidene halides. However, these barrier materials are not always completely effective for all the types of gases or liquids with which they come into contact. This is why work has been carried out to find barrier products which are more effective then those mentioned above. Liquid-crystal polymers (LCP) were proposed so as to be able to achieve almost complete impermeability over a wide range of gases and liquids.
Patent Application JP-09/76380xe2x80x94A discloses a container for storing frozen food products which can also be used for the preparation of microwave-oven food, which has gas-barrier properties and consists of plastic bilayer shells having a rim. The container is manufactured by injection-moulding an internal layer made of a structural plastic chosen from polyethylene, polypropylene, polyethylene terephthalate, polybutylene terephthalate and polycarbonate, and by overmoulding an external layer made of LCP. The rim has a welding surface made of structural plastic.
However, these known containers are small in size and poorly suited to industrial or motor-vehicle environments. In addition, their lifetime is generally very short and often does not exceed a few months. Beyond this period, they rapidly lose their barrier properties.
The object of the present invention is to provide hollow bodies which are impermeable to liquids or gases, are of various sizes, which may include large hollow bodies, which are robust and retain their mechanical and barrier properties over many years under severe environmental conditions, such as tanks used for storing fuel, particularly fuel tanks of internal combustion engines such as, for example, the tanks on board of motor vehicles.
For this purpose, the invention relates to a thermoplastic multilayer hollow body obtained by welding at least two moulded hollow parts together and at least one layer of which comprises a barrier material, in which the layer comprising a barrier material is an inner layer which comprises at least one liquid-crystal polymer (LCP).
The expression xe2x80x9cmultilayer hollow bodiesxe2x80x9d should be understood to mean a hollow body whose walls are formed from a stack of at least two layers of different materials.
The term xe2x80x9cfuelxe2x80x9d should be understood to mean any liquid or gaseous product or composition which can be used as a fuel and/or an oxidizer for supplying an internal combustion engine or a fuel cell.
The hollow bodies according to the invention are made of thermoplastic, that is to say a plastic which, under the influence of heat, melts or softens sufficiently to allow it to be formed. The term xe2x80x9cthermoplasticxe2x80x9d should be understood to mean any thermoplastic polymer, including thermoplastic elastomers, as well as blends thereof. The term xe2x80x9cpolymerxe2x80x9d should be understood to mean both homopolymers and copolymers (especially binary or ternary copolymers). Examples of such copolymers are, non-limitingly: random copolymers, alternating copolymers, block copolymers and graft copolymers.
The thermoplastic of one particular layer may be the only one in the multilayer structure of the hollow body. It may also be found in at least two separate layers of this structure. In either situation, a particular thermoplastic may be by itself or in the presence of other compounds chosen from additives of a wide variety of types, these being intended to modify the intrinsic properties of the thermoplastic and being well known to those skilled in the art. Examples of such additives are non-limitingly: fillers, stabilizers, plasticizers, lubricants, UV screens, etc. It is also possible to combine at least two different thermoplastics within a single layer of the structure. In this case, the layer may also possibly contain at least one of the abovementioned additives.
The hollow bodies according to the invention are impermeable to liquids and to gases; in other words, they have the ability to be able to retain the liquids and the gases in contact with their surface and to stop these materials migrating through their mass. In this case, the hollow bodies are impermeable to the gases and the aqueous or organic liquids liable to come into contact with them. Among the liquids, possibly including an inorganic or organic solute, are water, aliphatic and alicyclic alcohols, hydrocarbons and, in particular, petrol fuels for internal combustion engines and diesel fuel for diesel engines. Among the gases commonly encountered mention may be made of air, oxygen, carbon dioxide and fuel vapours. Several different liquids and/or gases as a mixture may also come in to contact with the articles to be impermeabilized.
According to the invention, the hollow body is obtained by welding at least two moulded hollow parts.
The term xe2x80x9cweldingxe2x80x9d should be understood to mean the operation of joining at least two articles together, which comprises a melting step carried out at least over one portion of the surface of each article, a step of bringing the articles into contact with each other and a final step of cooling the molten material back down to the solid state for the purpose of obtaining a composite assembly of articles which behaves, from the mechanical standpoint, as a single article.
The xe2x80x9chollow partxe2x80x9d refers to any part or article whose surface has at least one empty or concave portion. In particular, it refers to plastic shells which can be butt-welded.
The moulding technique used for manufacturing the hollow parts may be any moulding technique well known per se in the field of manufacturing plastic articles. For example, it may consist of injection moulding, compression moulding, injection-compression moulding; blow moulding or moulding by thermoforming. The term xe2x80x9cinjection mouldingxe2x80x9d should be understood to mean a moulding technique consisting in introducing molten plastic under pressure into a mould until the closed volume defined by the mould is completely filled, in cooling in order to set the hollow body and then in opening and removing the mould. As regards the technique of compression moulding, this consists in depositing plastic in an open mould and then in closing the mould and applying pressure for the time necessary to form the article which is then demoulded after the mould is opened. The technique of injection-compression moulding consists firstly in injecting plastic into an open mould and then in closing the mould and applying pressure, as in the technique of compression moulding. The technique of blow moulding consists in deforming a plastic article by pressure on the walls of a mould due to the action of a pressurized gaseous composition. Finally, the technique of moulding by thermoforming consists in impressing a permanent deformation on a plastic article by the action of heat. The technique of injection moulding has given good results.
The term xe2x80x9cbarrier materialxe2x80x9d should be understood to mean a material impermeable to liquids and gases, and in particular polymeric barrier resins.
According to the invention, the layer comprising a barrier material includes at least one liquid-crystal polymer.
The term xe2x80x9cliquid-crystal polymerxe2x80x9d (LPC) should be understood to mean a polymer whose molecules are in a fluid and ordered state over a range of temperatures intermediate between those which correspond to the solid state and those which correspond to the completely disordered melt (thermotropic polymer) or else a polymer capable of existing in the ordered state in solution (lyotropic polymer).
Preferably, the LCP used in the process according to the invention belongs to the class of thermotropic polymers. Such polymers generally comprise stretched chains or an alternation of rigid units and flexible units.